This invention relates to a medical device employing a therapeutic agent as a component thereof. For example, in an arterial site treated with percutaneous transluminal coronary angioplasty therapy for obstructive coronary artery disease a therapeutic antithrombogenic substance such as heparin may be included with a device and delivered locally in the coronary artery. Also provided is a method for making a medical device capable of localized application of therapeutic agents.
Medical devices which serve as substitute blood vessels, synthetic and intraocular lenses, electrodes, catheters, and the like, in and on the body, or as extracorporeal devices intended to be connected to the body to assist in surgery or dialysis are well known. For example, intravascular procedures can bring medical devices into contact with the patient's vasculature. In treating a narrowing or constriction of a duct or canal percutaneous transluminal coronary angioplasty (PTCA) is often used with the insertion and inflation of a balloon catheter into a stenotic vessel. Other intravascular invasive therapies include atherectomy (mechanical systems to remove plaque residing inside an artery), laser ablative therapy, and the like. However, this use of mechanical repairs can have adverse consequences for the patient. For example, restenosis at the site of a prior invasive coronary artery disease therapy can occur. Restenosis, defined angiographically, is the recurrence of a 50% or greater narrowing of a luminal diameter at the site of a prior coronary artery disease therapy, such as a balloon dilatation in the case of PTCA therapy. In particular, an intra-luminal component of restenosis develops near the end of the healing process initiated by vascular injury, which then contributes to the narrowing of the luminal diameter. This phenomenon is sometimes referred to as “intimal hyperplasia.” It is believed that a variety of biologic factors are involved in restenosis, such as the extent of the injury, platelets, inflammatory cells, growth factors, cytokines, endothelial cells, smooth muscle cells, and extracellular matrix production, to name a few.
Attempts to inhibit or diminish restenosis often include additional interventions such as the use of intravascular stents and the intravascular administration of pharmacological therapeutic agents. Examples of stents which have been successfully applied over a PTCA balloon and radially expanded at the same time as the balloon expansion of an affected artery include the stents disclosed in U.S. Pat. No. 4,733,665 (Palmaz), U.S. Pat. No. 4,800,882 (Gianturco), and U.S. Pat. No. 4,886,062 (Wiktor).
Also, such stents employing therapeutic agents such as glucocorticoids (e.g. dexamethasone, beclamethasone), heparin, hirudin, tocopherol, angiopeptin, aspirin, ACE inhibitors, growth factors, oligonucleotides, and, more generally, antiplatelet agents, anticoagulant agents, antimitotic agents, antioxidants, antimetabolite agents, and anti-inflammatory agents have been considered for their potential to solve the problem of restenosis. Such substances have been incorporated into (or onto) stents by a variety of mechanisms. These mechanisms involve incorporating the therapeutic agents into polymeric coatings and films, including hydrogels, as well as covalently binding the therapeutic agents to the surface of the stent.
For example, therapeutic agents have been dissolved or dispersed in a solution of polymer in an organic solvent. This is then sprayed onto the stent and allowed to dry. Alternatively, therapeutic agents have been incorporated into a solid composite with a polymer in an adherent layer on a stent body with fibrin in a separate adherent layer on the composite to form a two layer system. The fibrin is optionally incorporated into a porous polymer layer in this two layer system. The therapeutic agent, however, is incorporated into the underlying solid polymer. The overlying porous polymer layer provides a porous barrier through which the therapeutic agent is transferred.
Conventional methods of loading the therapeutic agent into a polymer, such as spray coating, do not provide high concentrations of therapeutic agents. Typically, upon spray coating a therapeutic agent onto a stent body, only about 2 percent of the spray is captured by the stent. This can be prohibitively expensive for therapeutic agents that are extremely costly and scarce, such as peptidic drugs.
Thus, what is needed is a medical device, preferably, a stent, having a porous polymeric material, typically a polymer layer in the form of a coating or film, with a therapeutic agent incorporated therein at sufficiently high concentrations that the therapeutic agent can be delivered over an extended period of time. Improved methods by which the therapeutic agent can be incorporated into the porous polymeric material with lower levels of waste are also needed.